1. Field of the Invention
The present invention relates to a target unit including a basic body and a target layer mainly comprising Si alloy disposed on an outer peripheral surface of the basic body by hard-facing method.
2. Prior Art
A planer-type target unit including a target layer made of Si alloy has been used in sputtering systems such as DC-diode sputtering system. In the sputtering process, atoms or groups of atoms are ejected from a surface of the target portion of the system as the result of heavy-ion impact to deposit a thin layer of metal on a glass, plastic, metal, or other surface of materials in vacuum.
However, the above target layer has several disadvantages generated through the sputtering process. For example, only 20% or less of the surface of the target layer is exposed to heavy-ion impact through the sputtering process, while the remaining part is being intact, so that the target layer cannot be used in the sputtering process efficiently. In the above systems, it has been also recognized that oblong shapes are difficult to use as a target, and further the planer-type target unit cannot be adapted to a continuous sputtering process.
Therefore, another type of target units has been proposed. One of the proposed target unit is shown in FIG. 4.
In this figure, a target unit 1 is in the form of a cylinder and comprises a basic body (herein after referred as a backing tube) of stainless steel and a target material (herein after referred as a target layer) 3 of Si alloy uniformity fixed to an outer periphery of the backing tube 2 by the hard-facing.
In the system using the cylinder-type target unit, about 80 to 90% of the surface of the target layer 3 can be subjected to heavy-ion impact through the sputtering process. Thus the system can be adapted to coating a material having a wide surface such as a glass surface, a mirror surface or the like. Therefore, the sputtering system using the cylinder-type target unit will extend its applicable sputtering range.
In spite of the above advantages, however, the target layer 3 of the cylinder-type target unit 1 is easily cracked or broken down as the result of the process of hard-facing. Because a degree of bonding strength between the backing tube 2 of stainless-steel and the target layer 3 of Si alloy is not sufficient to resist physical or mechanical stress such as heat shock or the like caused by the hard-facing process. Consequently, the target layer 3 can be easily pealed off from the backing tube 2.
It is noted that the expanding or contracting rate of the backing tube 2 is different from the one of the target layer 3 under the thermal condition. Therefore, the target layer 3 can be easily pealed off from the baking tube 1 during the sputtering process as a result of weakening the bonding strength between the backing tube 2 and the target layer 3 by thermal fatigue.